1. Field of the Invention
The present invention relates to a neutron flux mapping system for a nuclear reactor, and more particularly to a neutron flux mapping system for a nuclear reactor which has an improved architecture and an enhanced reliability while being efficient in terms of installation space and maintenance, and to which a substitution means is easily applicable when a failure of a part thereof occurs.
2. Description of the Related Art
A nuclear reactor typically includes 30 to 60 thimbles, depending upon the capacity thereof. In order to produce a neutron flux map along each thimble, a neutron flux mapping system using movable detectors is used. Such a neutron flux mapping system includes detectors, detector cables, drivers adapted to insert or withdraw respective detectors into or out of a core of the nuclear reactor, and path selector units adapted to guide each detector into a particular one of the thimbles. In order to measure neutron flux in a nuclear reactor, four sets of drivers having dedicated detectors, and path selector units are typically used. In accordance with operation of the path selector units adapted to guide the detectors of respective detector/driver sets, the four detectors of respective detector/driver sets can be selectively inserted into associated ones of the thimbles, the number of which may be 30 to 60.
Referring to FIGS. 1a and 1b, a conventional neutron flux mapping system is illustrated. The conventional neutron flux mapping system includes drivers 10, inlet detector guide tubings 11 each connected, at one end thereof, to an associated one of the drivers 10 while having a tubular shape to allow a detector to pass therethrough, and a path selector unit 30 connected to the other end of each inlet detector guide tubing 11. A detector cable, which carries a detector at a leading end thereof, is wound in each driver 10. In accordance with operation of each driver 10, the detector of the associated detector cable is inserted into the path selector unit 30 via the associated inlet detector guide tubing 11, and then is inserted into a selected one of the thimbles via the path selector 30.
As shown in FIG. 1b, the conventional path selector system 30 has a double layered architecture having upper and lower layers, at which four upper path selectors 31 and four lower path selectors 32 are arranged, respectively. The path selector system 30 may also have a triple layered architecture. The layers of the path selector system 30 are connected by a plurality of detector guide tubings. That is, detector guide tubings extend from each of the upper path selectors 31, and are distributed to respective lower path selectors 32.
In the above mentioned conventional neutron flux mapping system, each detector cable is inserted into a selected one of the thimbles via the associated upper and lower path selectors 31 and 32 in accordance with the associated driver 10, so as to achieve a remote neutron flux detection.
However, the above mentioned conventional neutron flux mapping system is complex in architecture and occupies an excessive space because the path selector unit 30 has a double layered architecture. For this reason, there are a difficulty in managing the system, and thus, an increased possibility of failure.
Furthermore, the interlayer distance of the path selector system 30, that is, the distance between the upper and lower path selectors 31 and 32, is short, thereby causing the detector guide tubings connecting the path selectors 31 and 32 to have a severe curvature. As a result, the detector cables reciprocating along the detector guide tubings may exhibit increased friction, thereby damaging the detectors, which are expensive. A failure may frequently occur in the drivers 10, which operate to insert or withdraw the detector cables. When a failure occurs in this system, a required repair should be carried out in the interior of a reactor containment vessel, that is, a highly radioactive region. In this case, there is a difficulty in performing tasks in that workers who perform tasks in the interior of the reactor containment vessel may be exposed to a large amount of radiation.
In addition, in the conventional neutron flux mapping system, the lower path selectors 32 are connected to thimble isolation valves (that is, the thimbles), respectively, in a 1:1 manner For this reason, if even one of the lower path selectors 32 fails, the overall system cannot operate normally because it is impossible to measure neutron flux through the thimbles associated with the failed path selector. In this case, there is a reduction in power generation rate or the plant should be shut down.
Meanwhile, each driver 10 should insert or withdraw the associated detector cable into or out of a desired thimble at a constant speed. However, the drivers 10 may frequently be rendered inoperable because of the structural problems, for example, the serious friction generated between the detector cables and the guide tubings, which is common in the conventional multiple layered path selector systems, cause the helical gear to exert excessive force which may result in wear or failure in the elements associated therewith. Furthermore, different stresses may be generated at each detector cable, depending on a variation in the insertion or withdrawal distance of the detector cable. For this reason, the expensive detector cable may frequently be damaged.